Terahertz electromagnetic waves in a frequency region of 0.1 THz to 30 THz lie in the intermediate area of high-frequency light and low-frequency radio waves, and the terahertz region remained largely undeveloped. Recently, since the laser beam technology has been developed, coherent generation and detection of terahertz electromagnetic waves is enabled, and because of the property that terahertz waves pass a material, various studies have been made in the security fields, such as food inspection and detection of poison concealed in envelopes, and the fields of, for example, tests for open faults in LSI, bimolecular imaging and dynamics and space stereoscope.
At present, the mainstream technique employed for detection of high-frequency electromagnetic waves at a normal temperature is a technique employing a Schottky barrier diode that is an electron transit element, and as the frequency is increased, more miniaturization is required, and accordingly, the sensitivity is lowered. According to the electron drift velocity, the usable frequency limit (the cutoff frequency) of a transistor is about 700 GHz.
On the other hand, in a case wherein a high-electron-mobility transistor (HEMT) is operated as a plasmon resonance element, the HEMT is functioned based on electronic polarization and vibration, without the dependence of real-space electron transport, and therefore, sensitivity will not be decreased even in a super high frequency range of the terahertz or higher. For the HEMT, since a two-dimensional electron gas (2 DEG) present in the electron transit layer has a very high electron density of about 1012 cm−2, the property as a fluid appears more remarkably than the movement of the electron. When such a two-dimensional electron fluid is externally excited, a collective vibrational electron wave, i.e., a plasma wave, is generated. When the plasma wave generated inside the structure of a resonator is quantized, a plasmon is obtained. While the drift velocity of electrons is about 107 cm/s at most, the velocity of the plasma wave is about 108 cm/s, which is greater by one to two digits than the drift velocity. Based on this fact, the plasmon resonance frequency generated due to the plasma wave is higher than the cutoff frequency of a conventional transistor, for which carrier transport is employed, and can reach the frequency level of several tens of THz. When the excitation of the two-dimensional plasmons is performed in this manner, detection of terahertz electromagnetic waves can be performed.
A mechanism for conversion between terahertz electromagnetic waves and plasma waves will now be described. Since an electromagnetic wave that propagates in free space is a transverse wave, while a plasma wave that is a collective vibrational electron wave is a polarized (longitudinal) evanescent wave oscillated in a non-radiation mode, all of the vibration components that move outward from the Plasmon area are attenuated, and essentially, the conversion between the two waves can not be performed. Further, it is required that the law of energy conservation and the law of momentum conservation be satisfied for performing this conversion; however, as described above, the velocity of the plasma wave is extraordinarily lower than the speed of light in vacuum (3×1010 cm/s), and in addition to this, the wavelength of the plasma wave is extraordinarily shorter than the wavelength of the electromagnetic wave. Since the vibration energy of the electromagnetic wave (photon) and the plasmon is proportional to the frequency, and the momentum is inversely proportional to the wavelength, when conversion is mutually performed between the plasmon and the electromagnetic wave that are regarded as those oscillated at the same frequency, the energy is conserved according to the law, but the momentum is not conserved. Also from this viewpoint, the performance of conversion is difficult.
The relationship between the wavenumber and the frequency for an electromagnetic wave and a plasma wave is shown in FIG. 1. A frequency k represented along the horizontal axis is the amount that is proportional to 1/λ and is related to the momentum. A frequency f represented along the vertical axis is the amount related to energy. As is apparent from FIG. 1, the law of conversion of momentum can not be established for a mode conversion at a specific frequency f3. In this drawing, a gradient c of a line for an electromagnetic wave is the speed of light of about 3×108 m/s. Further, a gradient vp of a line for a plasma wave is a value of about 105 to 106 m/s, which is proportional to the square root of an electron density n. The relationship in FIG. 1 is schematically shown, and the gradients of the two curves are different by about two digits.
In FIG. 2, a conventional terahertz electromagnetic wave detection device is shown that can resolve the above described problem to perform mode conversion. An HEMT structure is provided, which includes a substrate 201, an electron transit layer 202, an electron supply layer 203, a source 204 and a drain 205, and first group gates G1 and second group gates G2 are arranged. A gate length L1 for the first group gates G1 (the individual comb-like gates are called “fingers”) is shorter than a gate length L2 for the individual second group gates G2, and the gates of these groups are located between the source 204 and the drain 205 with the same spatial periodicity W. Since the first group gates G1 and the second group gates G2 are gratings having the periodicity W, and are interdigitated each other, these two groups of gates are called together “interdigitated dual grating gates”.
At this time, assume that a high voltage VG1 was applied to the individual first group gates G1, and a low voltage VG2 (<VG1) was applied to the individual second group gates G2. More electrons have gathered and been confined under the first group gates G1 than under the second group gates G2, and therefore, a difference in the electron densities n has occurred. As described above, the speed of the plasma wave is proportional to the square root of the electron density n. When the first group gates G1 and the second group gates G2 are under the different voltage applied states, electron plasma waves present below the individual gates become localized standing waves, for which the widths of the gates are employed as periodicity, and the resonant frequency of the fundamental standing wave can be represented by the following equation.
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                f          p                =                                            v              p                        iL                    ∝                                    n                        iL                                              (        1        )            where i denotes the interger of 2 or 4, and in a case wherein a symmetric boundary is present at both ends of the high electron density area below each gate, these two ends are employed as either nodes or anti-nodes, and therefore, the value of i is 2, or in a case wherein an asymmetric boundary is present, one of the two ends is a node (or an anti-node), while the other end is an anti-node (or a node), and therefore, the value of i is 4. As for the first group gates G1, since the electron density n is higher and the gate length L is shorter than those for the second group gates G2, a relationship is established representing that the resonant frequency of the first group gates G1 is sufficiently higher.
The periodicity of the resonant frequency of the plasma wave changes the line for the plasma wave shown in FIG. 1. Specifically, the line of the plasma wave is folded at the position of a wavenumber π/W that is the boundary of the Brilliouin zone due to the periodicity of the gates. It is apparent from FIG. 3 that, as the result of folding, there is a frequency at which the law of energy conservation can be satisfied, and the law of momentum conservation can also be satisfied. It can be said that the first and second group gates G1 and G2, alternately arranged with the same periodicity W, serve as antennas to enable the performance of mode conversion between the terahertz electromagnetic waves and the plasma waves.
The relationship in FIG. 3 is schematically shown, and it should be noted that, while the actual magnitude of the gradient vp is taken into account, multiple frequencies are generated at which both the law of energy conservation and the law of momentum conservation are satisfied.
Further, a case wherein different voltages are applied to the first group gates G1 and the second group gates G2 has been employed for the above description, and when one voltage is set higher than the other, the same analysis is established. For example, instead of applying a voltage, the second group gates G2 may be short-circuited, and a voltage may be applied only to the first group gates G1.
Furthermore, when the gate lengths differ for the first group gates G1 and the second group gates G2, regardless of whether the potentials are equal, the resonant frequency is also different, and periodicity occurs, so that the same analysis as in the above description is established.
In patent literature 1, a terahertz electromagnetic radiation device that employs the structure shown in FIG. 2 is disclosed.
With the above described mechanism, terahertz electromagnetic wave energy can be converted into plasma waves by a periodic antenna structure, i.e., plasma waves can be excited. Further, when on the contrary, plasma waves are excited by a specific external factor, conversion of the plasma energy into terahertz electromagnetic waves can also be performed. Conversion of terahertz electromagnetic waves into plasma waves is effective means as a terahertz electromagnetic wave detection mechanism, and conversion of plasma waves into terahertz electromagnetic waves is effective means as a terahertz electromagnetic wave generation mechanism. The two mechanisms for detection and generation of terahertz electromagnetic waves will now be described.
When terahertz electromagnetic wave energy is converted into plasma waves, and the plasma waves are excited, it is required that non-linearity of the plasma waves be effectively utilized again in order to detect terahertz waves. A terahertz electromagnetic wave detection device is a device, such as a Schottky barrier diode as a typical example, that outputs a direct-current voltage or a direct current in accordance with incident electromagnetic power of terahertz waves. The current-voltage characteristic of a diode is a non-linear characteristic that a current is exponentially increased, together with a voltage, in the forward direction. As shown in FIG. 4A, when a terahertz electromagnetic wave has entered a Schottky barrier diode, to which a predetermined positive direct-current bias VDC was applied in advance, a diode current is modulated, at the frequency of the incident electromagnetic wave, in consonance with an instantaneous voltage component Va sin ωt. At this time, since the current-voltage characteristic of the diode is non-linear, a distortion component shown in FIG. 4B occurs in the current of the diode, although the incident terahertz electromagnetic wave is a sine wave signal of a single frequency. As a result of the non-linear response, it is apparent that the direct-current component that indicates the temporal average value of the diode current has been fluctuated from the original value of the direct current. This change ΔITH depends on the electric power of the incident terahertz electromagnetic wave, and therefore, when the change of the direct current of the diode is measured, the incident electric power of the terahertz electromagnetic wave can be detected. For this process, a load resistor need only be connected to the diode to detect the fluctuation of the diode current as a change ΔVTHZ of a voltage generated at the two ends of the load resistor. As the non-linearity of the current-voltage characteristic is great, the detection sensitivity is increased.
In a case wherein terahertz electromagnetic waves are converted into plasma waves, the non-linearity of plasma waves can also be employed to provide the same detection mechanism as described above. At this time, in a case wherein plasma waves are excited for the electron transit layer of the above described transistor, the electrode portion of the transistor can extract a detection signal as a change of a direct current or a direct-current voltage, and according to the source-grounded structure where a source electrode is grounded and a drain electrode is connected to a power source via a load resistor, fluctuations of a drain electrode are obtained from the drain electrode. At this time, in order to generate a significant change of the drain voltage, i.e., in order to perform sensitive detection of terahertz electromagnetic waves, it is required that, at the drain electrode, the above described distortion component should be generated, with as a large magnitude as possible, for the plasma waves that were excited by absorbing the terahertz electromagnetic waves. The detailed theory for detecting terahertz electromagnetic waves based on plasma waves by employing a transistor with the simplest structure having a single gate is described in non-patent literature 5, and according to this description, the highest detection sensitivity is obtained in a case wherein a source electrode is electrically grounded and a drain electrode is employed as electrically an open end. In a case wherein both the source electrode and the drain electrode are grounded, a voltage change can not occur in the drain electrode, and thus, it is understood that the drain electrode should be electrically an open end. It is important here that an electrically different boundary condition (hereinafter referred to as an “asymmetrical boundary condition”) is provided for both ends (the source end and the drain end) of an area in an electron transit layer where plasma waves produce standing waves. It is apparent, from different viewpoints, that energy of terahertz electromagnetic waves is converted into direct current energy via plasma waves present under the asymmetric boundary condition.
Furthermore, in the electron transit layer of the transistor, temporal variations of electron densities in the spatial distribution constantly occur due to, for example, a thermal disturbance, or the presence of a direct current that flows from the drain to the source (microscopically, electron drift motion), or a photocurrent that is accompanied by photoelectrons and an electron hole pair generated by absorption of light. The collective vibrational electron wave that changes along the time axis is a plasma wave. Especially, in a case wherein a direct current flows between the source and the drain by the electron drift motion, and wherein the asymmetric boundary condition that both the source end and the drain end should be respectively a short-circuited end and an open end is established, the amplitude of the plasma wave is gradually increased by repeating multi-reflection at the drain end, and self-oscillation occurs. This is called a plasma instability. The principle of the plasma instability will now be described.
For simplifying the explanation, employ the ideal asymmetric boundary condition that the source end is short-circuited and the drain end is open. At this time, a case wherein the current component of a plasma wave reciprocally propagates between the source and drain is employed. The plasma wave is a charge density wave, for which localized fluctuations of charges generated by local variations in the electron density distribution, are shifted at a specific speed. Therefore, the current component of the plasma wave is provided by employing a product of localized variations eΔn of a charge density and a propagation speed vp of a plasma wave. Here, e denotes an elementary charge, and Δn denotes localized variations of the charge density. At this time, assuming a case wherein, in the electron transit layer, electrons move from the source to the drain at a uniform drift velocity vd by applying a direct current bias between the source and the drain, a current jF of a progressive wave that propagates from the source to the drain (this path is defined as a forward path) is provided as jF=eΔnF·vpF=eΔnF·(s+vd), where ΔnF and vpF represent Δn and vp at the pertinent time. On the other hand, a current component jB of the wave along the return path (a regressive wave) of the plasma wave that was fully reflected at the drain end (the ideal open end) is provided as jB=eΔnB·vpB=eΔnB·(s−vd), where ΔnB and vpB represent Δn and vp at the pertinent time. Since the plasma wave flows in the forward path while being superimposed with the drift current of electrons, the velocity of s+vd is provided (s is the velocity of the plasma wave), and since in the return path, the plasma wave moves backward against the drift current, the velocity of s−vd is provided. Since the electric current is maintained at the ideal open end before and after the reflection, jF=jB is established. Generally, the order employed for s is 108 cm/s and the order employed for vd is 105 to 106 cm/s, and since s>vd is established, ΔnB>ΔnF is established based on the condition of jF=jB. That is, after the reflection has been performed, the fluctuation of the charge density is increased. As the reflection is repeated, the fluctuation of the charge density is increased. At this time, when C (F: farad) denotes the capacitance of a capacitor arranged between the gate electrode and the electron transit layer, a relationship of Q=en=CVg is established for the capacitance C and a potential Vg between the gate and the electron transit layer, and therefore, the gate potential Vg changes in accordance with the fluctuation eΔn of the charge density. When the increase of the charge density continues in this manner, the fluctuation of the gate potential is also sequentially increased, and the oscillation state becomes unstable. This plasma instability was proved by M. Dyakonov and M. Shur in 1993 (see non-patent literature 4). This is also called the Dyakonov-Shur (DS) instability. The necessary condition for induction of the DS instability is asymmetry of the boundary condition (the condition for the source-end impedance and the drain-end impedance) of a plasma wave resonator (here, an electron transit layer).
As a conclusion for the two operation mechanisms, i.e., those for detection and generation of terahertz electromagnetic waves, so long as terahertz electromagnetic wave energy can be efficiently converted into direct current energy by performing conversion between terahertz electromagnetic waves and plasma waves, a very sensitive terahertz electromagnetic wave detection device can be obtained, and so long as direct current energy can be efficiently converted into terahertz electromagnetic waves, a very efficient, high-output power terahertz electromagnetic wave generation device can be obtained.